A World to Explore

Archive for May, 2011

Will Cary in front of the jet taking us to Israel. Challenging photo, I hope you know, with bright sunlight outside and reflective glass windows!

NEWARK, NEW JERSEY–Wooster junior Will Cary and I are departing very soon on a flight to Tel Aviv, Israel, and the start of our ten-day field trip to Israel. We will be exploring the Jurassic of southern Israel (the Matmor Formation in Hamakhtesh Hagadol — a favorite place for several Wooster geologists including Jeff Bowen, Meredith Sharpe, Sophie Lehmann, Elyssa Krivicich, Micah Risacher and Andrew Retzler (click the “Israel” tag to the right to see dozens of our blog posts). Our plan is the usual one: after arrival in Tel Aviv, we will drive south to Mitzpe Ramon, our Negev base, where we will meet Yoav Avni of the Geological Survey of Israel. We will then make the daily drive to the Large Makhtesh to measure strata and collect specimens. We want to concentrate on a crinoid-rich fauna near the middle of the formation. My dream is to find a significant number of rare bryozoans and maybe a few brachiopods as well. Since our goal is paleoecological reconstruction, all that we find is data.

Boarding a flight to Israel is always an adventure in itself. The gate for the Tel Aviv flight is set apart from the others at the end of a concourse. It has its own separate security procedures, and once you are in the waiting area you are discouraged from leaving. Guards with automatic weapons stay ready in the background, visible but not obvious. Each passenger walking up the ramp to the plane is looked square in the eyes by a grim Israeli security officer. (I give an innocent and naive scientist’s smile back.)

Fifteen hours or so from now I expect we will arrive in the little town of Mitzpe Ramon in the afternoon sunshine. I can feel the gravel under my boots already.

In advance of my next field trip to Israel (watch this space!), our highlighted fossil this week is the scleractinian coral Microsolena, a genus named by the French naturalist Jean Vincent Félix Lamouroux in 1821. The specimen above was collected from the Matmor Formation in Hamakhtesh Hagadol in the Negev Desert. It is Callovian in age, specifically the athleta Zone. (I know a lot of details about this area!) This coral is thus roughly 160-165 million years old.

Scleractinian corals appeared first in the Triassic and are the primary coral in today’s oceans. Unlike their extinct Paleozoic cousins, scleractinians have skeletons made of aragonite rather than calcite. Aragonite is relatively unstable and easily dissolves over geological time. Our specimen above has been replaced with the more stable calcite. This means that the exterior is preserved well enough to identify to the genus level, but details in the interior necessary for species determination have been recrystallized beyond recognition.

A nice oyster is still attached to the coral surface. Oyster shells are made of calcite and so are usually preserved very well. You can also see holes in the coral made by boring bivalves and given the name Gastrochaenolites. One of the bivalve borings is in a raised lump of the coral (center top of the image). This is reaction tissue built by the coral in response to the invading bivalve, a clear indication that some of the boring took place while the coral was alive. Most of the corals in the Matmor Formation are heavily bored by bivalves.

Field view of cross-sections of bivalve borings (some with bivalve shells still in them) in a scleractinian coral in the Matmor Formation.

The Matmor Formation is exposed only in the cavity of Hamakhtesh Hagadol. Here it is about 100 meters thick and consists mostly fossiliferous marls and sponge-coral patch reefs. (One of the previous Fossils of the Week is a thecideide brachiopod attached to corals like the one above.) The Matmor sediments were deposited on a shallow marine ramp near the Middle Jurassic equator. It is this equatorial deposition that makes the Matmor such an interesting subject for paleoecological analysis. Most other described Jurassic faunas are in Europe and North America, and they were all formed under more temperate conditions.

Of course, there’s always the classic documentary, like Andrew’s Dacite documentary. The documentary is closely related to the mockumentary – see Will’s petrology project on how to make a thin section as an example:

This week’s specimen is a piece of obscure paleontological history, although it represents a “fossil” that was for a short time one of the most prominent in the world. In 1864, the uber-geologist Charles Lyell claimed it was “one of the greatest geological discoveries of my time”. Charles Darwin cited it in his fourth edition of On the Origin of Species as the most critical find yet to show the rise of life from single-celled ancestors. Yet it is only a metamorphic rock with no evolutionary relevance whatsoever. Wooster obtained a sample of this material sometime in the nineteenth century, early enough in the story that it was cataloged into the fossil collection and labeled with its original name: Eozoön canadense.

In 1864, two major figures in North American geology met to discuss a set of layered rocks found just west of Montreal and now known to be about 1.1 billion years old (Proterozoic Eon). William E. Logan (1798-1875), the Director of the Geological Survey of Canada, showed John William Dawson (1820–1899), Principal of McGill University, specimens he believed represented evidence of Precambrian life. Dawson not only agreed that these were fossils, he published a description in 1865 announcing them as “one of the brightest gems in the scientific crown of the Geological Survey of Canada”. He named the fossil Eozoön canadense — the dawn animal of Canada.

Eozoön canadense; (A) as illustrated by Dawson; (B) the holotype in the U.S. National Museum of Natural History. (Scale bars = 1 cm; figures from Schopf, 2000.)

Dawson concluded that Eozoön canadense was the test of a single-celled protistan known as a foraminiferan — even though it is staggeringly larger than any foraminiferan ever known. Eozoön was an immediate hit, attracting the attention of Darwin, Lyell and others intensely interested in finding the deepest roots of the fossil record.

Oddly enough, Dawson had an opposite opinion of the importance of Eozoön. He was an anti-evolutionist anxious to discredit Darwin’s ideas that were quickly sweeping the scientific world. He wrote:

“There is no link whatever in geological fact to connect Eozoön with the Mollusks, Radiates, or Crustaceans of the succeeding [rock record] … these stand before us as distinct creations. [A] gap … yawns in our imperfect geological record. Of actual facts [with which to fill this gap], therefore, we have none; and those evolutionists who have regarded the dawn-animal as an evidence in their favour, have been obliged to have recourse to supposition and assumption.”

In other words, Dawson thought his new fossil would be the death of evolutionary theory because it opened up an unbridgeable “gap” between “primitive” and “advanced” animals. Ironically, at the same time Darwin was grateful to at last have a single-celled fossil at the base of the family tree.

Eozoön canadense had a short and contentious life as a fossil. It was immediately challenged as inorganic by many scientists. In 1879, a German zoologist named Karl Möbius published a study showing that whatever it is, Eozoön canadense has no relationship with the foraminiferans and probably no other organism. Dawson held firm to his beliefs. The final blow came in 1894 when two geologists found Eozoön in boulders of marble shot out of Mount Vesuvius. Apparently the “fossil” is a metamorphic rock made of layers of white calcite and green serpentine. Dawson was unmoved and was actually working on yet another Eozoön paper when he died in 1899.

So our Fossil of the Week turns out not to be a fossil at all, and the name Eozoön canadense is now a nomen nudum — a “naked name” signifying a taxonomic mistake. At one time, I imagine, the geologists at Wooster were pleased to have a fragment of the oldest evidence of life. Now I treasure our specimen as a connection to the early passions of our science.

The fossils above are about as simple as fossils can be. They are internal molds (sediment-fills) of conical shells that were made of the carbonate mineral aragonite. The aragonite shells dissolved away after death and burial, leaving the cemented sediment behind. While not complex, these fossils have historic value in paleontology. They represent an extinct group called hyoliths, and they were found where the very first hyoliths were described by Eichwald in 1840: the Middle Ordovician of Estonia. I collected them on my first field trip to the Baltic States in 2006. (My original interest in picking them up, by the way, was in the faint squiggles on the outside of the molds — a trace fossil known as Arachnostega.)

Hyoliths are rather common in some rock sequences. They are among the earliest shelly fossils known, found in the lowest Cambrian rocks (about 540 million years old). They peaked in abundance in the Cambrian and lived throughout the Paleozoic Era, finally going extinct at the end of the Permian Period (around 250 million years ago).

Reconstruction of a living hyolith (by "Smokeybjb" via Wikipedia).

For as many hyolith fossils we have, they remain an enigmatic group. They had conical shells, usually a bit flattened, with a hinged lid (operculum) over the open end. Extending from the space between the operculum and cone were two calcareous rods called helens (a name deliberately chosen so as not to evoke a particular function). Some rare hyolith fossils show evidence of internal features, including muscle scars and a twisted intestinal tract. We still can’t definitely place them in a particular animal group, though, and even their life habits are obscure. They probably were deposit-feeders (digesting organic material from seafloor mud), but the support for this is speculative.

The hyoliths of Estonia tell us one more thing: they are different enough from other hyoliths around the world to show us that the paleocontinent of Baltica likely had its own biogeographic province. In other words, Baltica was isolated as an island continent during the Middle Ordovician (around 460 million years ago), much like Australia today.

Baltica is the small green continent shown on this global reconstruction of the Cambrian (public domain from Wikipedia).